A victory by Nicole Branagh and Elaine Youngs later Sunday would give the United States half of the final four and a guarantee of at least one medal.

Trailing 20-15 in the first, Brazil survived three set points before Larissa served it into the net to give the set to the Americans. May-Treanor and Walsh, three-time defending world champions, who have not lost since a domestic tour match last August.

Thus ends the partnership between Larissa and Ana Paula, who were forced together when Larrisa's regular teammate, Julianna, injured her anterior cruciate ligament and pulled out on the eve of the Olympics. The two had never even practiced together until the morning of their first match.

On Saturday, both American men's teams advanced to the round of 8.

Sean Rosenthal and Jake Gibb beat Spain 26-24, 21-17 to earn a quarterfinal matchup with defending gold medalists Ricardo and Emanuel of Brazil. Todd Rogers and Phil Dalhausser beat Switzerland and will play the German team of David Klemperer and Eric Koreng.

Rosenthal pulled out his left-handed spike at a key point in the match, when the Americans led 18-17 in the second set having already won the first.

After a recovery with his right hand, Rosenthal got the pass back from Gibb and found himself in a position where his conventional spike wouldn't do. So he slammed the ball down with his left, a switch few players are able to make.

Instead of being tied, the Americans had a two-point lead, needing two points to win. They held on to beat Raul Mesa and Athens silver medalist Pablo Herrera Allepuz of Spain.

"I don't know how he does half the stuff he does out there," Gibb said. "He's as good with his left as I am with my right."

Rosenthal, who doesn't do anything else with his left hand, learned how to hit lefty when he hurt his right arm as a teenager. A beach bum who wanted to keep playing volleyball, he taught himself to play lefty by banging the ball against his garage door.

He said he doesn't think about using his left arm, which he's done just a few times in four Olympic matches so far.

"I just let it tell me when it's going to hit," he said.

Rosenthal also had nine digs on defense - as many as the other three players on the court combined.

"It was one of the best performances I've ever seen out of my partner. He absolutely carried me," Gibb said. "He showed what he could do out there today, on the biggest stage of all."